Liquid membrane separation processes constitute an emerging technology with unusually widespread applications that include the separation of hydrocarbons, wastewater treatment, recovery and purification of metal ions, oil well control, and biomedical applications such as the artificial kidney, removal of phenolic and other toxins from blood, the treatment of drug overdose, and the timed release of enzymes and drugs.
Thus, how to detoxify industrial waste water by removing its wide variety of contaminants, such as organic and mineral acids, phenolic compounds, amines, ammonia, various metal ions and salts, including phosphates, chromates, nitrates, nitrites, sulfates, and chlorides, has been a recurring and widespread problem. Emulsion liquid membrane processes have been shown to be effective in such applications.
Although the principle of the invention will be primarily illustrated with reference to emulsion or unsupported liquid membranes, it will be evident that the ideas involved may be easily adapted to the so-called immobilized or supported liquid membrane. The latter is formed when a liquid is impregnated in the pores of a porous solid for mechanical support (Noble and Way, 1981, Ch.1).
Unsupported or emulsified liquid membrane systems consist of a solute receptor phase, emulsified or encapsulated as fine droplets of perhaps 1 .mu. in diameter, in another immiscible liquid. Globules of this emulsion, often having diameters of 0.1-5 mm, are dispersed in a third liquid phase containing the solute to be removed. The latter is called the continuous or donor phase, and solute travels from the external donor to the internal receptor phase through the intervening immiscible liquid which, because of its selective barrier function, is referred to as a liquid membrane. Ideally there is no direct contact between the encapsulated and continuous phases, which are commonly miscible in each other. The insulation of these two phases from each other depends on preserving the stability of the intervening immiscible liquid membrane, accomplished by the presence of an appropriate surface active agent, initially in the membrane phase. To maintain a solute concentration gradient across the membrane the solute is often caused to react chemically with a reagent in the receptor phase, giving a product that is insoluble in the membrane liquid, so preventing its countertransfer back to the donor phase. "Carrier" agents are sometimes added to the membrane phase to solubilize otherwise insoluble solutes, thereby facilitating their transport across the membrane as complexes.
FIGS. 1 and 2 illustrate the terminology and experimental procedure for an organic membrane phase, and FIG. 3 specifically shows the removal of phenol or acids from wastewater as merely one example of such processes.
However, a major stumbling block permeates the whole liquid membrane field; it lies in preserving the stability of the liquid membrane against rupture or breakage, thereby permitting leakage of the internal or receptor phase back into the external or donor phase, thus nullifying the solute separation already achieved. Such membrane rupture is promoted by the agitation shown in FIG. 2 and/or by inappropriate formulation of certain components, such as insufficient surfactant, or membrane liquids of too low viscosity. Two remedies for this problem are currently in use, but they are known to be far from ideal, as follows:
The first current remedy: Increase the concentration of stabilizing surfactant in the membrane phase.
Unfortunately this reduces the rate of solute transfer by inhibiting any internal motion within the emulsion globules, and by setting up both mechanical and adsorptive barriers to solute transfer at the interfaces between the membrane and the inner and outer phases.
In addition, surfactants have been found to decrease the rate of interfacial chemical reactions like that in FIG. 3. (Nakashio et al., 1988).
The second current remedy: Increase the viscosity of the membrane phase.
Numerous workers (e.g.; Terry et al, 1982; Frankenfeld et al, 1976; Yang and Rhodes, 1980) have found that membrane stability increases but transfer rates decrease substantially as the membrane viscosity is increased. This is to be expected from the Eyring-Stokes-Einstein relation for molecular diffusivity in Newtonian liquids (Skelland, 1985, pp 54-5): ##EQU1## Evidently a ten or twenty-fold increase in membrane viscosity decreases the molecular diffusivity to around 1/10-1/20 of its value in low viscosity membranes. This has been widely observed in the liquid-membrane context (e.g. Kataoka et al, 1989).
Thus the current remedies for liquid membrane instability both tend to nullify its advantage of high permeability, the latter resulting from short transfer path, strong selectivity, and high diffusivity.